The nondestructive testing of solids with the aid of ultrasonics is of relatively recent origin. Reference in this connection may be made, for example, to an article by Byron E. Leonard and C. Gerald Gardner in a survey titled NONDESTRUCTIVE TESTING, NASA SP-5113, published 1973 by the Technology Utilization Office of National Aeronautics and Space Administration, pp. 27ff. An evaluation system using a transducer for emitting the supersonic waves and detecting their echoes is diagrammatically illustrated in FIGS. 3-19 on page 36 of the publication.
In commonly owned German printed specification No. 25 35 019, laid open Feb. 10, 1977, there has been disclosed a technique of ultrasonically testing spherical bodies with the aid of a support forming a spherically concave seating surface in which a body to be tested is cradled, the support and the body under test being immersed in a coupling liquid which forms a film or cushion between the body and its seat; an ultrasound generator disposed below the support emits waves through the latter and the film of coupling liquid toward the center of the body which is also the center of curvature of its seat. The body is rotated during the test by one or more nozzles trained substantially tangentially upon its nadir, i.e. upon the lowest point of the interspace separating it from the support.
While the system described in the German specification operates generally satisfactorily in many instances, we have found that it does not uniformly respond to flaws in the interior and near the surface of the tested body.
We attribute this shortcoming to the fact that the system is most responsive to echoes from the interior of the body and is less sensitive to those originating near its surface.
It has already been proposed to explore the structure of a test body at or near its surface with the aid of several relatively offset ultrasound generators; see the magazine ULTRASONICS of November 1973, pp. 247 and 249. This, however, does not solve the problem of nonuniform sensitivity to structural defects at different depths.